知识学人
because和for强调“为什么”，说话人希望大家关注原因
Because引表示直接原因，for表示间接原因，或者推断的根据。
for引导的原因状语从句一般只能在句中。
如：
Kate spoke quietly because she didn’t want Ali to hear.
Kate 平静地说话，因为她不想让Ali听到。
I’m sure it rained last night, for the ground is wet.
我确信昨晚下雨了，因为地面是湿的。
since和as也是“因为”，但只是附带说明原因，说话人希望大家关注结果。
二词差别不大，但since的理由比as更充分一些。
Since everyone is here, let’s start.
既然大家都来了，我们就开始吧。
I really want Ellen to come as I want to hear about her trip to France.
我真的很想让Ellen 来，因为我想听一听她去法国的旅行。
例句:(注意观察下面例句中，主从句的先后顺序，以及逗号的使用。)
Because I opened Lisa’s letter, she’s angry with me.
因为我打开了丽莎的信，她生我的气。
Lisa is angry with me because I opened her letter.
丽莎因为我打开了她的信而生我的气。
The days are shorter and shorter, for it’s December.
白天越来越短了，因为是十二月。
As Mike was in Spain, he decided to watch Flamenco dance.
Mike在西班牙时，他决定去看弗拉门戈舞。
Mike decided to watch Flamenco dance as he was in Spain.
Mike决定在西班牙观看弗拉门戈舞。
We had a video conference (视频会), since half of our colleagues were in the US.
我们有一个视频会议，因为我们一半的同事都在美国。
Since half of our colleagues were in the US, we had a video conference.
因为我们一半的同事都在美国，所以我们开了一个视频会议。
乐学一百
虽然都表原因，各自侧重不同。
Because：侧重原因
【例句】：She spoke quietly because she did not want to wake up the baby.
【翻译】：她小声得说话，因为不想吵醒孩子。（直接的因果，不想吵醒孩子导致说话声音小）
【例句】：Because I am disappointed, I want to go.
【翻译】：因为我失望了，所以我想离开。
As：侧重结果，由于听者所知道的原因
【例句】：Don’t tell me bad things in the morning as it sets the mood for the rest of the day.
【翻译】：大早晨的，别告诉我坏事，因为早晨的心情决定了一整天的心情。
【例句】：As it is raining outside, you’d better stay at home.
【翻译】：外面在下雨，你最好还是呆家里吧。
Since：侧重结果，通as，比as更正式，与其也更强。
【例句】：They are expensive, since their quantity is small.
【翻译】：物依稀为贵。
【例句】：Since people can do many things at home via internet, it is not necessary for them to find a job outside.
【翻译】：既然人们可以在家通过网络做很多事，所以他们无须出去工作。
For：表示推断的原因，或解释说明，只能放句末。
【例句】：He must be at home, for it is raining.
【翻译】：他一定在家，因为下雨呢。 下雨不是必须在家的原因，却是推测的依据。
【例句】：I want to see you, for I miss you.
【翻译】：我想见你，因为我想你。
英语学人
because口语常用，后面接句子，说明直接原因是什么。the car crashed because the driver was careless.直接原因是因为驾驶员不小心，所以汽车撞坏了。
for后可接句子和短语，表示推测的原因。for做介词后面跟名词或名词性词组。 for做连词连接句子，表推测原因，或原因对主句的补充说明。I was angry with him for being late .我很生气，因为他迟到了（for后面加的是短语）It must have rained,for the ground is all wet.(for后面加句子表示推测原因)天一定下过雨，因为地上全湿了。根据地上湿了推测下雨
since和as说明的原因理由不是直接的，重要的而是大家都知道的，一般放句首。翻译成既然。As you are tired, you'd better rest.既然你累了，那就休息吧。since light travel faster than sound ,we see lightening before we hear the thunder.因为光比声走的快，所以我们先看到闪电，后听到雷
